1. Field of the Invention
This invention relates to a projection device of an erect one-to-one magnification system of short conjugate for use in a copying apparatus or the like.
2. Description of the Prior Art
It is known that the conjugate from the object plane to the image plane may be reduced by a so-called ommeteal optical system in which a plurality of element lens systems for projecting a part area of an object onto a predetermined part area on the image plane are arranged in a plane perpendicular to the optical axis in a predetermined direction, namely, lengthwise of the slit in a slit exposure type copying apparatus, thereby making the entire body the copying apparatus compact. Element lens systems each comprising three lenses arranged in the direction of the optical axis are known from U.S. Pat. Nos. 3,592,542 and 3,584,952. The middle one of the three lenses arranged in the direction of the optical axis acts as a field lens and, by the imaging actions of the two lenses on the opposite sides of the field lens, a part area of an object is projected onto a predetermined part area on the image plane and as a whole, the slit area of the object is projected onto the corresponding slit area on the image plane at erect one-to-one magnification. Briefly describing the imaging actions, the object point is imaged as an inverted intermediate image just at the center of the length of the optical axis of the element lens system by the lens nearest the object side (referred to as the first lens) and thereafter, is re-imaged on the image plane by the lens nearest the image plane side (referred to as the third lens), and finally, an erect one-to-one magnification image is formed. That is, generally, such optical system is a symmetrical optical system. The first lens and the third lens suffice for the imaging system, but to prevent any loss of the quantity of light, namely, to suppress the aperture eclipse, a second lens having a field action is disposed near the intermediate image. On the intermediate image plane, a mask is provided to limit the view field in each element lens system, and an area corresponding to the mask view field is imaged on the final image plane and this is superposed lengthwise of the slit so that when scanning is effected, the exposure amount distribution integrated in the scanning direction is uniform lengthwise of the slit.
However, in such apparatus, it is necessary to accurately set the shape and size of the mask, the arrangement pitch in the lengthwise direction of the slit and the relative position of the mask to the lenses in the element lens systems, in conjunction with the arrangement of the lenses of each element lens system in the lengthwise direction of the slit, and in fact, this is considerably difficult.
By the workmanship of this mask and the relative relation between the mask and the lenses during the setting, the irregularity of exposure amount in the lengthwise direction of the slit is determined and thus, the optical performance for the entire apparatus depends greatly on the mask.
Therefore, if the conventional mask can be omitted and yet the function as previously described can be provided, it will be very desirable from the viewpoint of manufacture.
Incidentally, U.S. Pat. No. 4,215,933 shows that an element lens system comprises three lenses and that a third lens is provided at a position deviated from the intermediate point between the lens near the object and the lens near the image plane, but in this patent, a mask is provided intermediate of each element lens system so that the light intensity distribution on the image plane has an oblique boundary of about 80.degree. with respect to the arrangement direction with the light intensity distribution of the adjacent element lens system, and this suffers from the problem as previously noted.